1. Field of the Invention
The present invention relates to a technique of acquiring/reproducing the reflection characteristics of an object and, more particularly to a gonio characteristic reproduction technique of acquiring/holding/reproducing the local gonio reflection characteristics of an object.
2. Description of the Related Art
Along with recent development of CG (Computer Graphics) techniques, it has become possible to generate a CG image with almost the same quality as that of an actually captured image. CG images are used in a wide range of fields including not only movie and game industries but also product design and commercial photos.
There is conventionally known a method of using BRDF (Bidirectional Reflectance Distribution Function) data of an object to generate a CG image. This method is implemented by a technique of generating BRDF data of an object using a mathematical model in addition to a technique of actually measuring the BRDF characteristics of a subject and generating BRDF data using actual measurement values.
There is also known a technique of using, as input data, a BTF (Bidirectional Texture Function) indicating the distribution of small BRDFs on an object surface to be rendered, in order to improve the reality of a CG image to be generated. This technique also allows reproduction of the texture of a rendering object.
As represented in a geometric form shown in FIG. 4, however, BTF data is data storing reflectances for the zenith angle (Sin) and azimuth 4 in) of incident light and the zenith angle (Bout) and azimuth (Out) of exit light. Therefore, the amount of data is enormous, and it is thus necessary to compress the data amount to enable processing at practical cost.
In Japanese Patent Laid-Open No. 2007-219715, for example, the amount of BTF data is reduced by measuring BTF data of a subject and that of a white plate, and then approximating the BRDF of each pixel by a cosine function. Furthermore, in Japanese Patent Laid-Open No. 2007-140820, a table R(u, v) indicating reflection characteristics of a BTF is defined for each combination of four angles θL, φL, θV, and φV. Some of all the pixels are set as representative pixels, and the remaining pixels are set as general pixels. Contents of the table R(u, v) are used intact for the representative pixels. For each representative pixel, rows and columns associated with φL, and φV of the table R(u, v) undergo rotation/shift to derive a plurality of variations of the table, and a derived table that approximates the BTF data of the pixel best is substituted for its table R(u, v), thereby reducing the amount of BTF data.
If, however, there is minute unevenness on the surface of a subject, the normal to each pixel varies from the macroscopic normal to a specimen plane. If the receiving angle is changed, an angle at which a highest intensity is obtained is different for each pixel. The technique described in Japanese Patent Laid-Open No. 2007-219715 simply approximates the BTF data of each pixel by the cosine function, thereby disabling to correctly reproduce a shift of the peak position of each pixel.
The technique described in Japanese Patent Laid-Open No. 2007-140820 considers a shift of a peak position and performs rotation/shift to derive a plurality of variations of the table for each representative pixel, thereby performing approximation. It is, however, difficult to approximate a BTF image of a subject with high accuracy.
The present invention provides a technique of readily measuring a BTF, and generating high precision BTF data using a small amount of data, in order to perform CG rendering at practical calculation cost.